Metals such as zinc, iron, chromium, zirconium, manganese, cobalt, nickel, titanium, and molybdenum in their oxide form, and phosphates thereof, have been used in metal pretreatment processes for almost a century, serving as an excellent base for applying a paint top coat, and also providing some protection against corrosion without paint. For aluminum pretreatment, hexavalent chromium is very popular, and its performance has been basically unparalleled by other known non-chrome alternatives. Unfortunately, because of the carcinogenic characteristic of hexavalent chromium, its uses are increasingly restricted in Europe and the United States. Besides health and environmental concerns, chromium adds financial burden to the aluminum pretreatment industry, for example the high expense in treating liquid waste discharge to remove it.
To address the above environmental and health-related issues related to hazardous metals, by this invention a typically metal-free, corrosion-resistant pretreatment coating is provided, being applicable to aluminum and its alloys, as well as zinc and its alloys, and magnesium and its alloys, as well as other metals. Thus, the coating may be applied to galvanized iron and steel components as well as to aluminum, magnesium and the like.
The coating of this invention is derived from a typically aqueous precursor solution, typically by dip-coating or spraying onto metal surfaces. The coating subsequently forms a cross-linked and typically hydrophobic film after a short duration of drying and crosslinking. The coating on metal exhibits excellent corrosion resistance, high thermal stability, strong adhesion to the metal substrate and to paint top coatings, if present, and typically exhibits a thickness of only about a micron or less. Corrosion testing results show that aluminum panels coated with such a submicron coating of preferred materials of this invention, without a paint top coat, can tolerate corrosive environments as described in ASTM B117, for up to 168 hours and more. To the contrary, uncoated aluminum panels begin to corrode in about six hours under this test. As a paint primer, the submicron coating of this invention provides excellent paint adhesion between metal and paint for up to about three thousand hours or more in salt spray tests, while untreated panels show high degree of paint loss in the same tests. The definition of “primer” here is two fold. First it refers to a pre-paint treatment. Also, it can refer to an underlying coating beneath a top coating.